When cells of a strain of Arthrobacter are incubated in light in the presence of a photosensitizing dye, synthesis of a single protein is enhanced to a remarkable extent. At 25 degrees, the optimal growth and induction temperature, about 30 minutes are required to achieve the maximal rate of synthesis. Electron microscopy of the bacterial cells treated under these conditions showed an impressive increase in the extent of piliation. This study will examine the mechanism of the induction. The dyes which generate singlet oxygen in the light, seem to act at the cell surface. Alterations in the cell membrane will be examined because of the well-known oxidation of unsaturated fatty acids acids, also converts cis to trans double bonds. This type of change is expected to markedly alter the fluidity of the cell membrane. Preliminary experiments in which the cells are preincubated at low temperatures suggest that decreasing the fluidity of the membrane may be involved in the induction process. Experiments are also in progress to purify the induced protein from 2 M NaC1 extracts of the cells. The protein has a marked tendency to aggregate, in the cold or upon removal of the NaC1, in a form that is difficult to dissociate. Upon purification experiments will be designed to examine the aggregation in order to obtain information on the mechanism of assembly of pili.